You have 4 equations is 4 unknowns. Let F = number of fish, D = number of dogs, Pf = price of a fish, Pd = price of a dog:

F(Pf) +D(Pd) = 96

D = 3F

F x Pf = 30 +D(Pd)

Pf=10.40 +Pd

Now to work through these four equations:

1. Put the value for D from equation 2 into equation 1. This reduces the problem to 3 equations in 3 unknowns:

F(Pf) + 3F(Pf) = 96

F(Pf) = 30 +3F(Pd)

Pf=10.4+Pd

2. Put the value for Pf from the third equation into the first two:

F(10.4+Pd) + 3F(Pd) = 96

F(10.4 + Pd) = 30 + 3F(Pd)

3. Subtract the 2nd equation from the first:

3F(Pd) = 96-(30+ 3F(Pd)); so F xPd = 11

4. Put F(Pd) =11 into the first equation:

10.40 F + 11 +3(11) = 96

Which yields F = 5

Now you can back substitue this into the previous equations to get the values for Pd, Pf, and D.